1. Field of the Invention
This invention relates to an angular velocity sensor utilizing a sensor element (vibrator) made of a piezoelectric material and to an angular velocity sensing system utilizing the angular velocity sensor to detect angular velocity, and is used, for example, as an angular velocity sensor in a gyroscope.
2. Description of the Related Art
Mechanical rotor-based gyroscopes have long been used in the inertial navigation systems of aircraft and ships. Although these mechanical rotor-based gyroscopes are excellent in stability and performance, they are not suitable for incorporation in small-scale equipment because of their large size and high cost.
This has led to the recent increasing application of small vibrating gyroscopes of intermediate accuracy. The vibrating gyroscope utilizes an angular velocity sensor which uses a piezoceramic to vibrate a sensor element and a separate piezoceramic provided on the sensor element to detect voltages produced by Coriolis-induced vibrations in proportion to angular velocity.
An angular velocity sensor using a quartz crystal sensor element is taught, for example, by Japanese patent laid-open publication No. 4-504617.
A conventional angular velocity sensor using a quartz crystal will be explained with reference to FIGS. 12 to 14. FIG. 12 is a perspective view of the prior-art angular velocity sensor, and FIGS. 13 and 14 are sectional views taken along lines B--B and C--C in FIG. 12.
As shown in these figures, the angular velocity sensor 60 consists mainly of a tuning-fork sensor element 61 having two arms 40, 41 and a base 42. The arms 40, 41 are formed of quartz crystal.
The arm 40 on the left side in the figures is provided with drive electrodes 43, 44, 45, 46 and sensor electrodes 51, 52, 53, 54. The arm 41 on the right side is provided with drive electrodes 47, 48, 49, 50 and sensor electrodes 55, 56, 57, 58.
Drive voltages are applied to the drive electrodes 43-50 to vibrate the arms 40 and 41 in the plane of the tuning fork (the plane in which the arms vibrate), i.e., in the directions of the arrows -X and X (along the X axis) shown in FIG. 12. At this time, if the angular velocity sensor 60 is rotated at an angular velocity .omega. about the longitudinal axis of the arms 40, 41 indicated by the arrow Y (the Y axis), Coriolis forces F proportional to the angular velocity .omega. are produced in the directions indicated by the arrows -Z and Z (along the Z axis) orthogonal to the X axis. The Coriolis force F is expressed by EQU F=2.multidot.M.multidot..omega..multidot.V
where M is the mass of the arms and V the vibration velocity.
The Coriolis forces F excite a new vibration in the Z-axis direction of the arms 40, 41 and the new vibration produces voltages at the sensor electrodes 51-58. The direction and magnitude of the angular velocity .omega. produced in the angular velocity sensor 60 can be determined by detecting these voltages by means of a detection circuit.
The symbols + and - in FIGS. 13 and 14 indicate the polarity of the drive voltages applied to the drive electrodes 43-50 and the polarity of the voltages produced at the sensor electrodes 51-58.
In the so-configured prior-art angular velocity sensor, the axis of the rotation whose angular velocity is to be detected must extend in parallel with the longitudinal direction of the two arms of the tuning fork (the Y axis in FIG. 12). This limits the degree to which the thickness of the angular velocity sensor can be reduced in the direction of this axis of rotation.
Another problem with this angular velocity sensor is the overall complexity of its electrode configuration caused by providing the drive electrodes at an upper portion of the arms 40, 41, the sensor electrodes at a lower portion thereof and the means for connection with these electrodes at the base 42 of the tuning-fork sensor element 61.
Since this arrangement requires vacuum deposition or sputtering steps for forming the sensor electrodes by use of a sensor electrode mask and then forming the drive electrodes by use of a drive electrode mask, it increases the cost of fabrication.
A still further drawback of the so-configured angular velocity sensor is that as a result of mechanical coupling therein a slight leak output voltage is produced at the sensor electrodes solely by the vibration caused by the periodic application of voltage to the drive electrodes, even when no angular velocity is being experienced. Since the phase of this leak output voltage is the same as that of the angular velocity sense voltage, the sense output includes the leak output voltage superimposed on the angular velocity sense voltage.
The angular velocity sensing system utilizing the conventional angular velocity sensor of this type has no means for compensating for this leak output voltage. Since the detection of the angular velocity is therefore made based on changes in the output voltage from the angular velocity sensor including the superimposed leak output voltage, the detection accuracy is markedly degraded.